Designing and commissioning of a two stage cyclone-spiral separation unit at the Zarand coal washing plant

Ash content of concentrates of cyclone-spiral units at the Zarand coal washing plant was higher than the allowable limit (11%) mainly. The main reason was due to the presence of particles smaller than 150 µm containing high amount of ash. The removal of this size fraction by installing a screen with aperture of 0.5 mm increased the quality of the concentrate. The stream passing the screen (removed fraction) used to be sent to the cyclone-spiral unit at the flotation section for further processing. Since the spiral concentrate of this unit had low quality (ash content of 18%) it was sent to the screening unit. The under screen stream used to be sent again to the cyclone-spiral unit at the flotation section. This procedure created a non-beneficial circulating load with the amount of 56% of the spiral concentrate. By modification in the dewatering process, this amount was reduced to 5%. In order to increase the quality of the final concentrate, a two-stage separation unit where the spiral concentrate instead of entering the screening unit was fed to another spiral was installed. Finally, by determining optimum values of 40 for solids concentration and 2 t/h for the feed to spiral, the amount of final concentrate and capacity were increased by 11 and 14 t/h, respectively. Furthermore, the final spiral concentrate ash decreased from 18 to 11%.